Many devices include displays for displaying images, text, and other content to a reader. Examples of such displays include cathode ray tubes (CRTs), liquid crystal displays (LCDs), electrophoretic displays (EPDs), light emitting diode displays (LED displays), and the like. Different types of displays have different components, configurations, and principles of operation for converting digital image data into the displayable image and displaying the image. An LCD, for example, must be illuminated from the front or back by an optical system. An EPD and other types of “eInk” and “electronic paper” displays reflect and absorb light incident on the display panel to create the image; thus, the EPD can be lit by ambient light and/or by a front light of the device.
Some devices can automatically adjust display properties and perform other actions to account for ambient light in the device environment. Where an illumination source is used, such as in a backlit LCD or a frontlit EPD, an intensity of the illumination may be varied based at least in part on the ambient light. For example, in sunlight, a backlight on an LCD may be increased while a frontlight on an EPD may be decreased in the same conditions. These devices can include ambient light sensors that detect and measure levels of ambient light incident on the device. Most current ambient light sensors may be inadvertently occluded during use, or may result in a chassis with one or more penetrations. An exemplary implementation that overcame these drawbacks is described in U.S. Pat. No. 8,933,916.
Additionally, various illumination sources may suffer performance variations that may be due to degradation over time and with use or variations in light output caused by variations in temperature. Such performance variations may be perceptible by users, thereby decreasing the functionality or user enjoyment of the device.